There are various combustible self-producing gases. For example, these are a coal bed gas (also referred to as a Coal Mine Gas and simply expressed as CMG) contained in coal beds, a biomass gas resulting from fermentation and putrefaction of general wastes or agricultural and livestock wastes, and so on. Especially, since coal beds in which coals are buried in large amount are existing all over the world, there is a large amount of CMG on the earth. When coal beds are mined for obtaining coal, CMG is captured and drained in advance for securing safety during mining work. The amount of CMG and contents of combustible component (most of which is methane) of CMG vary with a lapse of time. The variation in the content of the combustible component of the gas means that the gas has various calories.
Actually, all CMG is released to atmosphere around coal mines, or in the neighborhood of residential areas, the gas of high calorie (high-calorie gas) is collected and utilized in the city as a city gas, while the gas of low-calorie (low-calorie gas) is released to atmosphere without utilization, because the low calorie is insufficient to use as a city gas. In addition, the amount of self-producing high-calorie gas varies, and therefore there is a need for a huge gas storage facility keeping a large amount of the high-calorie gas for the purpose of stable supply.
Conventionally, there have been proposed technologies for utilizing the self-producing gases such as CMG for electric power generation. For example, there has been disclosed a system in which a gas engine generator capable of generating a predetermined amount of electric power using the self-producing low-calorie gas and another gas engine generator capable of generating a predetermined amount of electric power using the high-calorie gas (normal city gas) are combined to thereby allow power generation to be maintained while switching supply of the gas to these generators and their operating states (e.g., see Japanese Laid-Open Patent Application Publication No. 2002-202006). This system is intended to omit huge gas tanks that can store the self-producing gas by additionally using the normal city gas.
However, since the calorie and the amount of CMG constantly vary irregularly, controlling the system and procedure to fully utilize CMG becomes more complex. Specifically, since an electric power is generated by the gas engines, the number of gas engines that may start and stop according to variation in the amount of gas supply, must be well controlled. This unavoidably causes specified gas engines to repeat start and stop so frequently, which may lead to shortened lives of the gas engines, and unstability of power distribution network. Furthermore, stable supply of the city gas is essentially needed, and therefore, in order to realize the above-described system in coal mining areas or landfill sites of wastes, such large-scale infrastructure as a city gas supplying system is required to be prepared.
Meanwhile, there has been proposed a technology in which electric power is generated by a gas turbine using CMG as fuel, and the resulting carbon dioxide as exhausted gas is fed into coal beds and fixed therein together with air free from oxygen (e.g., see Japanese Laid-Open Patent Application Publication No. 2003-74372). However, this document does not disclose a system or a method for maintaining stable power generation by efficiently consuming low-calorie gas regardless of perpetual variation in the amount and calorie of the self-producing gas, which are the basic characteristic of the CMG.